The Food and Drug Administration has issued a warning to five makers of pure powdered caffeine about the health risks associated with their products.

In the warning, the FDA called pure powdered caffeine “potentially dangerous products that have already resulted in the known deaths” of two people.

A high school senior in Ohio died last year after taking pure powdered caffeine and a 24-year-old man in Georgia died just a month later after being in a coma caused by his use of the drug.

The agency issued letters to SPN, LLC, Purebulk, Inc., Kreativ Health, Inc., Hard Eight Nutrition, LLC, and Bridge City Bulk, warning that their products are “dangerous and present a significant or unreasonable risk of illness or injury to consumers.”

But the FDA’s ability to take action against these companies is limited compared to its jurisdiction with “conventional” food and drug products. Since powdered caffeine is marketed as a dietary supplement, it is somewhat unregulated. While the FDA regulates marketing of dietary supplements, manufacturers are not required to get FDA approval before making or selling their products.

This isn’t the first FDA statement on pure powdered caffeine: The agency also issued a warning to consumers last year, recommending that people avoid the products entirely.

In the latest letter, the FDA states that just one teaspoon of pure powdered caffeine is equivalent to the amount of caffeine found in about 28 cups of regular coffee.